Void stuff, as it was described by the Tenth Doctor, was a type of background radiation that clung to those who travelled through the Void. The Doctor was able to show its presence on himself and all those who had travelled to or from Pete's World with his 3-D glasses. A full opening of any breach into the Void would violently pull the matter back in, along with anything it was clung to.
However the fact that void stuff existed was a direct contradiction to the Doctor's earlier statement that the void contained 'nothing', including 'no radiation' and therefore could not exist in the first place. It is possible, however, that "void stuff" is actually a form of dark matter, not truly tangible and is what the void is composed of.